In a conventional thread cutting process the workpiece is held in place while a cutting tap is passed through the workpiece to form the threads and then the tap is backed out of the workpiece in the reverse direction. This type of process is normally used for creating threads in many fastener products.
However, there are several instances where internal threads cannot be created on components by the usual thread cutting process. For example, the American Petroleum Institute has prescribed standards for making sucker rod coupling tubes that do not permit the internal threads to be created by a cutting process. These sucker rod couplings are short lengths of internally threaded tubes used for connecting together components of longer lengths.
A thread forming or rolling tap method has been used to form the internal threads on these sucker rod couplings. The thread forming process is similar to swaging where a tap is used to compress the inner surfaces of the tube to form the threads. In contrast to the conventional cutting process, the tap cannot be backed out of the tube once the threads have been made during the initial forward pass. The prohibition against backing the tap out of the part is due in large part to the desire to prolong the life of the precision tap.
While admittedly increasing the tool life the inability to back the tap out of the part does complicate the operational steps necessary to complete the process. After the tap initially passes through the part the tap must be released from its tool holder and the workpiece must be separated from the tap. Many of the sucker rod coupling tubes now in use have been made by a substantially manual process in which workmen physically grasp the end of the tap protruding from the part and pull the rest of the tap completely through the workpiece.
Some effort has been made to automate this process. In one known machine each tube is horizontally fed in an end-to-end fashion into a work station having a set of V-shaped jaws for clamping each tube during the thread forming process. Separate drive cylinders are used to independently control the feed of the parts to the work station, the clamping of the workpieces, and the clamping of the tap while the threaded part is slid rearwardly over the shank of the tap.
Machines of this type have serious drawbacks. They are designed to tap only one part at a time yet they require a multitude of separate drive cylinders which must operate in properly timed relationship. They do not have the capability of readily adapting themselves to new part sizes without requiring a major change. The feeding of the parts into the work station requires that the tubes be oriented in an end-to-end relationship. Such an orientation requirement may unduly increase the complexity and cost of the conveyor equipment necessary to transport the tubes to the threading work station.
The present invention is directed to solving one or more of these problems.